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Properties of direct summands of modules. (English) Zbl 0659.16016
L. Fuchs posed [in his “Infinite abelian groups” I (1970; Zbl 0209.055)] the problem of characterizing the abelian groups in which the intersection of each pair of direct summands is a direct summand. This property (called the summand intersection property or SIP) has been studied, for arbitrary modules, by G. Wilson [Commun. Algebra 14, 21-38 (1986; Zbl 0592.13008)], who also characterized modules with SIP over principal ideal domains. In this paper, modules M are studied with the property that the sum of any pair of direct summands of M is a direct summand of M (summand property or SSP). Characterizations of rings are obtained in terms of the SSP of their modules, and, for a module M, the SSP (or the SIP) of M is studied through properties of $$End_ R(M)$$. Furthermore the author studies the conditions when a given module, which can be written as a direct sum of indecomposable modules, has the SSP (or the SIP or both). Some particular cases are investigated.
Reviewer: F.Loonstra

##### MSC:
 16D70 Structure and classification for modules, bimodules and ideals (except as in 16Gxx), direct sum decomposition and cancellation in associative algebras) 16S50 Endomorphism rings; matrix rings 16W50 Graded rings and modules (associative rings and algebras)
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##### References:
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